Rare cells, such as circulating tumor cells (CTCs), are found in peripheral blood samples in very low numbers. For example, CTCs in the blood of cancer patients are found in a concentration of less than 100 cells per ml of blood. Because CTCs are known to be shed from a primary tumor and to cause metastasis, rapid detection and identification of these rare cells have significant impact not only on monitoring cancer progress, but also on expanding the understanding of cancer metastasis. Rare cells also include fetal cells that are found in maternal blood. Various types of rare cells have been isolated and detected by several techniques based on fluorescence microscopy, microfluidic sorting, flow cytometry, genomic analysis, and immune-magnetic assays.
Conventional and advanced microscopy techniques are important diagnostic tools associated with the detection of a variety of targets such as viruses, bacteria, and mammalian cells. However, the costs of these techniques and their relative complexity challenge reliable and feasible implementation in rural areas and inner cities.